High sensitivity electrical switching circuit

ABSTRACT

A switching circuit for use with an electrical appliance, and comprising first and second normally open switch terminals and a high sensitivity impedance measuring circuit coupled to the first and second switch terminals for measuring an impedance therebetween, and producing at an output thereof a switching signal if the impendance is lower than a predetermined threshold of 500 MΩ. The appliance is coupled to the output of the impedance measuring circuit so as to be responsive to the switching signal. The circuit finds particular application for controlling and protecting electrical appliances operating from an a.c. electrical means supply, in which case one of the switch terminals is connected to a virtual ground connection which is electrically floating with respect to a ground feeder of an electrical mains supply. In the event of electrical contact between the virtual ground connection and the other switch contact, an apparent ground fault is produced, which gives raise to a small current which is detected by the impendance measuring circuit so as to produce a switching signal which may then be used to operate a suitable relay for controlling the appliance.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the national stage under 35 U.S.C. 371 ofPCT/IL98/00464, filed Sep. 25, 1998.

FIELD OF THE INVENTION

This invention is related to electrical switching and control circuits.

BACKGROUND OF THE INVENTION

Electrical and electronic devices are frequently required to be operatedand controlled remotely via suitable switching and control circuits. Inthe case of high power devices operating on relatively high voltages,such as devices intended for operation on the main electricaldistribution network, the resulting control and switching currents canbe significant reaching several hundred milliamperes. When currents ofsuch magnitudes flow through long cables, there is generated asignificant voltage across the cable whose magnitude is proportional tothe specific impedance, or resistivity, of the cable. This, in turn,gives rise to energy wastage and increased operating costs. The voltageacross the cable can be reduced by employing lower gauge (i.e. thicker)cables whose resistivity is correspondingly lower, but this results inthe control and switching cables being bulky as well as expensive.

Published PCT application no. WO 95/31028, in the name of presentinventor, discloses a detector for monitoring the integrity of a groundconnection to an electrical appliance having live and neutral terminalsfor feeding current to the appliance from respective live and neutralfeeders of an electrical supply having a ground point for connecting tothe ground terminal of the appliance. The detector comprises adifferential comparator circuit for comparing a voltage at the neutralconnection with a voltage at the ground terminal of the appliance andproducing a fault signal if a difference therebetween exceeds apredetermined threshold. A switching device is connected in at least oneof the live and neutral connections so as to be opened by a relayoperatively coupled to the detector and responsive to the fault signalproduced thereby.

WO 97/36358 published on Oct. 2, 1997, in the name of the presentapplicant, discloses a specific application of such a detector for usewith electrical appliances which are either ungrounded or whose groundconnection is impaired. Thus, in accordance with WO 97/36358 there isprovided a protection device for use in conjunction with an electricalappliance having an electrically conductive outer casing and whichprotects against the casing becoming “live” regardless of the state of aground connection associated with an electrical supply to which theappliance is connected, the electrical appliance having live and neutralterminals for feeding current to the appliance from respective live andneutral feeders of the electrical supply, said device comprising:

a virtual ground connection which is electrically floating with respectto said ground connection of the electrical supply, said virtual groundconnection for electrically coupling to the casing of the electricalappliance instead of the ground connection of the electrical supply, and

a ground impedance measuring circuit for measuring an impedance betweeneither the live or neutral terminals and the virtual ground connectionof the appliance and producing a fault signal if said impedance fallsbelow a predetermined threshold.

In effect, the protection circuit disclosed in WO 97/36358 employs thedifferential comparator subject of WO 95/31028 to compare the voltagesbetween live and neutral and a floating ground connection such that anydiscrepancy between the measured voltages is indicative of a groundfault. There are several major advantages of the use of a floatingground connection as opposed to a regular ground connection. First,there is no danger of a person effecting electrical contact with thefloating ground connection becoming electrocuted since there is noreturn path through actual ground for the fault current. Second, byusing appropriate resistors in the differential comparator circuit, the“fault” current required to register an imbalance may be reduced tofractions of a nanoampere (i.e. less than 10⁻⁹ ampere) as distinct fromthe milliamperes associated with conventional ground fault detectorcircuits. Yet another advantages is, of course, the lack of dependenceon a reliable ground connection.

The use of a floating ground connection per se is known. Thus, EP 695105 discloses a protection device for use with an appliance having aground connection 33, which might be the metal casing of the electricalappliance, which is connected via a resistor R1 to a protection circuitcomprising elements EC2, EC3, EC4 and OC1 so that if the leakage currentflowing through R1 exceeds a certain threshold, this protection circuitprovides a trigger to a relay coil RL1 so as to open the primary switchcontacts SW1 and SW2. Thus, the protection circuit produces a faultsignal if the ground impedance falls below a predetermined threshold.

Likewise, FR 2468430 discloses a protection device wherein, as inabove-described EP 695 105, the principle of operation is that, in theevent of a ground fault, there will be a leakage current flowing throughthe ground connection and the magnitude of this leakage current isemployed in order to provide a trip signal for the main currentbreaker(s). In the case of a regular ground fault between either thelive or neutral feeders and GND, the resulting ground leakage currentwhich flows through the virtual ground connection effects adequateprotection.

However, in the event of a short-circuit between the live and neutralconnections, there will under normal circumstances be no ground leakagecurrent and therefore the circuits described in EP 695 105 and FR2468430 will afford no protection. This is a very serious drawbackbecause short-circuit faults represent a significant risk of fire.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a high sensitivity switchingcircuit for allowing remote switching of an electrical device via a pairof wires carrying negligible current whereby high gauge cables ofminimal cross-sectional area may be employed without resulting inunacceptable ohmic losses.

In accordance with a broad aspect of the invention there is provided aswitching circuit for use with an electrical appliance, the switchingcircuit comprising:

first and second normally open switch terminals,

a high sensitivity impedance measuring circuit coupled to the first andsecond switch terminals for measuring an impedance therebetween, saidhigh sensitivity impedance measuring circuit producing at an outputthereof a switching signal if said impedance is lower than apredetermined threshold of 500MΩ;

the appliance being coupled to said output so as to be responsive tosaid switching signal.

The invention finds particular application for controlling andprotecting electrical appliances operating from an a.c. electrical mainssupply. For such applications, one of the switch terminals is connectedto a virtual ground connection which is electrically floating withrespect to a ground feeder of an electrical mains supply. In the eventof electrical contact between the virtual ground connection and theother switch contact, an apparent ground fault is produced. The groundfault gives rise to a small current which is detected by the impedancemeasuring circuit so as to produce a switching signal which may then beused to operate a suitable relay for controlling the appliance.

A sufficiently strong fault signal may be generated upon contact betweenthe virtual ground connection and ground by a human being having a bodyresistance typically in the order of several thousand ohms. By suchmeans, human contact with one or both of the switch terminals issufficient to register a “ground fault” thus producing the requiredswitching signal. If desired, one or both of the switch terminals may beconnected to a metal touch plate such that momentary contact therewithby a human being results in the generation of the required switchingsignal.

Alternatively, both switch terminals can be mutually floating withrespect to ground whilst exhibiting a very high contact impedance ofseveral hundred MΩ. Shorting the switch contacts, even by means offinger contact, reduces the impedance to below the threshold of theimpedance measuring circuit, thereby produce the switching signal. Inthis context, it is to be understood that “shorting” means lowering thecontact resistance between the two switch contacts to less than thepredetermined threshold of 500MΩ. The resulting current which then flowsinto the impedance measuring circuit may be as low as severalnanoamperes.

According to a preferred embodiment of the invention, the impedancemeasuring circuit comprises a differential comparator circuit forcomparing a fraction of the voltage between the live and neutralconnections of the electrical supply with a voltage at the virtualground connection. A fault signal is produced if a difference betweenthe two voltages exceeds a predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, there will now be described a switching circuit forconnection to an electrical supply feeder, by way of non-limitingexample only and with reference to the accompanying drawings, in which:

FIG. 1 shows schematically the principles of the invention;

FIG. 2 is a schematic representation of a switching circuit connected toan incoming electrical supply for remote switching of an electricalappliance;

FIG. 3 shows schematically a detail of a differential comparator circuitused within the switching circuit;

FIG. 4 is a schematic representation of a high-sensitivity switchingcircuit according to an alternative embodiment of the invention; and

FIG. 5 is a schematic representation of an application using low costsurface wiring for use with the switching circuit according to theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a system 10 comprising an electrical supply 11 having liveand neutral supply rails phases 12 and 13, respectively and a groundrail, GND. Connected to the live rail 12 is a voltage regulator 14 forproducing at an output 15 thereof a regulated low voltage d.c. supply.The output 15 of the regulator 14 thus constitutes a high d.c. rail,whilst the neutral supply rail 13 constitutes a low d.c. rail and willbe referred to thereby in the subsequent description.

Connected across the high and low d.c. rails is a first voltage dividercomprising a pair of resistors 16 and 17 whose common junction 18 isconnected to a first output 20 of a differential voltage comparator 21.A second input 22 of the differential voltage comparator 21 is connectedto a common junction 23 of a second voltage divider comprising a pair ofresistors 24 and 25. A free end 26 of the resistor 25 is connected to atouch plate and constitutes a switch terminal.

An output 27 of the differential voltage comparator 21 is connected tothe base of an NPN bipolar junction transistor 28 whose emitter isconnected to the low d.c. rail and whose collector is connected to thehigh d.c. rail 15 via a suitable switching element K which may be anelectromagnetic or solid-state relay. An appliance 29 has powerconnections which are energized via a normally open switch contact 30which is closed under control of the switching element K in knownmanner. Thus, the switching element K is analogous to known contactorswhich allow high voltage devices to be switched via low voltage controlor switching circuits.

A person 31 who touches the switch contact 26 whilst standing on ground,GND allows a small electric current to flow through the second pair ofresistors 23 and 24 through him to OND. As a result, there is producedan imbalance between the voltages at the two inputs 20 and 22 of thedifferential voltage comparator 21 whose output 27 thus goes high. Whenthis happens, the bipolar junction transistor conducts therebyenergizing the relay K and closing the normally open switch contact 30thus supplying current to the appliance 29. It will, of course, beappreciated that the same principle can equally well be applied tointerrupting the current to the appliance 29 by substituting a normallyclosed switch contact for the normally open switch contact 30.

Within the context of WO 97/36358, the person 31 causes a ground faultand the switch contact 26 which is electrically floating with respect toGND constitutes a virtual ground connection. These terms will becomeclearer from the following description of a specific arrangement.

By Ohm's Law, the impedance of an electrical device is equal to thevoltage across the device divided by the current flowing therethrough.Thus, if there is no earth leakage to the switch contact 26, then theimpedance between the switch contact 26 and the live supply rail 12 isextremely high, assuming that the resistors 23 and 24 have suitably highvalue resistances. Specifically, if the phase voltage is 220 V and themaximum permitted safe leakage current is 0.9 mA, then the impedance istypically in excess of 250 KΩ. However, in the event of an apparentground fault, such that the leakage current to the switch contact 26rises, the impedance drops accordingly. The impedance between the highd.c. rail and the junction 23 thus serves as a measure of whether thereis an earth leakage to the switch contact 26.

Since the switch contact 26 is floating relative to GND, the integrityor lack of integrity of the ground connection to the neutral feeder 13of the electrical supply 11 is no longer significant. Specifically, theprovision of a sound ground connection GND or the lack of such provisionis not relevant; in either case fast operation of the relay K is ensuredwithin no more than several milli-seconds. It will also be appreciatedthat by using suitably high value components for the resistors 16, 17,23 and 24, an apparent ground fault may be produced even when the“ground” leakage current is negligible: for example in the order ofnanoamperes. This order of sensitivity is hardly applicable when thedetector circuit is used as a regular ground fault protection device astaught in WO 97/36358. However, it is admirably suitable to theapplication of a high sensitivity touch switch, and the resultingnegligible current flow allows high gauge control cables having verysmall cross-sectional areas to be employed thus reducing the cost ofsuch cables as well as their bulk.

Referring now to FIG. 2, there is shown a circuit diagram of a touchswitch 35 based on the above-described principles. Connected across thelive and neutral feeders 12 and 13 of a mains electricity supply 11 isan isolation transformer 36 having a tapped secondary winding 37 forstepping down a primary voltage of 110 or 220 V approximately 10 Vacross respective taps. A d.c. bridge rectifier 38 is connected to anappropriate one of the tapped secondary windings via a selector switch39 and produces an output voltage of approximately 12 V d.c. across ahigh voltage rail 40 and a low voltage rail 41 constituting,respectively, supply and ground rails.

The live and neutral inlets 12 and 13 are also connected viacorresponding switches 42 and 43 to live and neutral socket outlets 44and 45, respectively to which the appliance 29 shown in FIG. 1 isconnected As shown, there is also provided a ground socket under 46 towhich an outer metallic casing of the appliance 29 may be connected, ifrequired. Such connection is not, however, mandatory since unlike thecircuit described in WO 97/36358, the touch switch 35 is not used tosafeguard against actual ground faults but rather is used for remoteswitching of the appliance 29.

The switches 42 and 43 constitute switching devices which aresimultaneously operated by a relay coil 48 (constituting a main feederswitching means which may be a contactor) connected between ground andthe collector of an NPN bipolar junction transistor 49. An emitter ofthe NPN bipolar junction transistor 49 is connected to ground and a basethereof is connected via a resistor 50 to a first normally open switchcontact 51 a of a two-pole changeover switch operated by a relay coil52. The relay coil 52 constitutes an “actuator” having a first pole 53 aand a first normally closed switch contact 54 a and having a second pole53 b and corresponding second normally open and closed switch contacts51 b and 54 b, respectively.

The first pole 53 a of the changeover switch is connected to thepositive low voltage d.c. supply rail 40 whilst the first normallyclosed switch contact 54 a is connected to one end of a red indicationlamp 55 whose other end is connected to ground. Likewise a greenindication lamp 56 is connected between ground and the first normallyopen switch contact 51 a. The second pole 53 b of the changeover switchis connected via a rectifier diode 57 to the one end of the relay coil52, a second end of which is connected to the positive low voltage d.c.supply rail 40 via a detector circuit depicted generally as 58 anddescribed in greater detail below with reference to FIG. 3 of thedrawings.

A filter capacitor 59 is connected across the relay coil 52. Likewise,for the sake of completeness, a filter capacitor 60 is shown connectedbetween the base of the bipolar junction transistor 49 and ground.Connected between the live and neutral inlets 12 and 13 is a voltagedivider comprising a pair of resistors 61 and 62 whose common junctionis coupled via a current limiting resistor 63 to one input of adifferential comparator within the detector 58 and whose second input isconnected to ground. Thus, the differential comparator compares afraction of the voltage between the live and neutral inlets 12 and 13 tothe voltage on the virtual ground connection 46.

Referring to FIG. 3 it is seen that the detector 58 comprises a lowvoltage d.c. rail 61 connected to the d.c. supply rail 40 shown in FIG.2. A 6 V regulator 62 is connected between the low d.c. voltage rail 61and ground such that there exists a regulated 6 V d.c. voltage between astabilized voltage rail 63 of the regulator 62 and ground.

A first voltage divider comprising resistors 64 and 65 is connectedbetween the stabilized voltage rail 63 and ground and has a commonjunction connected to a first inverting input (pin 2) of a dualcomparator 66 so as to provide a reference voltage signal which isoffset from the ground potential by a fixed amount. The common junctionof the voltage divider shown in FIG. 2 is also connected via a resistor67 to the first non-inverting input (pin 3) of the comparator 66. A 6 VZener diode 68 is connected in series with a current limiting resistor69 between the stabilized voltage rail 63 and ground, between which isconnected a smoothing capacitor 70.

An output 71 (pin 7) of the comparator 66 is connected to the base of anNPN bipolar junction transistor 72 (constituting a “normally openswitching circuit”) whose emitter is connected to ground and whosecollector is connected to the second normally open switch contact 51 bshown in FIG. 2. The output 71 is also connected via a resistor 73(shown in FIG. 2) to ground.

The rectifier diode 57 which is connected across the relay coil 52shunts any high back e.m.f. generated by the coil 52 and thus avoidsdamage to the bipolar junction transistor 72.

In order to increase the reliability of the detector 58, the comparator66 comprises dual comparators sharing a common output and connected inan analogous manner to the arrangement described above. The dualcomparator may be constituted by an integrated circuit such as NationalSemiconductor's LM193 series.

The operation of the detector 58 is as follows. Under normal conditions,there is a large imbalance between the voltage at the virtual groundconnection 46 and the fractional feeder voltage which are respectivelyfed to the input pins of the comparator 66. Consequently, the outputvoltage of the comparator 66 is high so that the base voltage of thebipolar junction transistor 72 is high and the bipolar junctiontransistor 72 conducts. In this condition, the bipolar junctiontransistor 72 functions as short circuit between its emitter andcollector such that the normally open switch contact 51 b is connectedto ground.

The high voltage rail 40 is connected via the resistor 50 to the base ofthe bipolar junction transistor 49. The bipolar junction transistor 49thus conducts, allowing current to flow through the relay coil 48 whichenergizes and closes the switches 42 and 43, thereby connecting theinlet terminals 12 and 13 of the supply 11 to the corresponding socketoutlets 44 and 45. At the same time, the high voltage rail 40 isconnected to the green indication lamp 56 which thus illuminates andprovides a visible indication that the appliance 29 is energized.

If now an operator touches the virtual ground connection 46 constitutingthe touch plate of the switch contact 26 shown in FIG. 1, an apparentground fault is produced such that the differential voltage seen by thecomparator 66 falls and its output 71 goes low. The transistor 72 isthus cuttoff and its collector is no longer connected to ground.Consequently, the voltage applied to the normally open switch contact 51b (which is closed when the appliance 29 is energized) is no longer atground potential and so the relay coil 52 de-energizes and thechangeover switch contacts revert to the “fault” state shown in FIG. 2.In this state, the high voltage rail 40 is connected to the redindication lamp 55 which illuminates and provides a visual indicationthat the appliance 29 is now de-energized. At the same time, since theswitch contact 51 a is no longer connected to the high voltage rail 40,the voltage applied to the base of the bipolar junction transistor 49goes low and the bipolar junction transistor 49 is cutoff. Thisde-energizes the relay coil 48 thereby tripping the supply between theinlet terminals 12 and 13 of the adapter 20 and the corresponding outletterminals 44 and 45 thereof.

There is thus provided in accordance with the invention a highlysensitive touch switch which is responsive to negligible current flowfor allowing remote operation and control of an electrical appliance.

Furthermore, since the ground connection of the installation, ifpresent, is in any case isolated from the appliance 29 in that it isquite distinct from the virtual ground connection employed thereby,there is no longer any danger of electric shock if the actual ground ofthe installation becomes live owing to a breakdown in insulation betweenthe live and ground feeders.

It will also be appreciated that, since the impedance of the applianceis negligible in comparison with the nominal impedance of the virtualground loop (whose threshold is set to be in the order of 250,000MΩ),the impedance of the virtual ground loop can be measured between thevirtual ground connection and either the live or neutral feeders.Assuming a supply voltage of 230 V, such calibration causes a leakagecurrent as low as 0.009 μA to cause operation of the switch 35.

It will also be appreciated that, whilst in the preferred embodiment thetouch switch serves to de-energize the appliance, it can equally well beemployed to energize the appliance from an initial de-energized state.This can easily be achieved by reversing the initial states of theswitch contacts 53 a and 53 b or using a contactor having normallyclosed contacts which open when the bipolar junction transistor 49becomes saturated. Alternatively, a pair of touch switches can beprovided: one for energizing and the other for de-energizing theappliance. Yet a further possibility is to replace one of the bipolarjunction transistors 49 or 72 by a bistable multivibrator (flip-flop).Each time the touch switch 35 is touched (thus creating an apparentground fault), an output of the bistable multivibrator changes stagethus alternatively energizing and de-energizing either one of the relays48 or 52.

It is also to be understood that use of the virtual ground connection 46as the ground socket outlet as shown in FIG. 2 is illustrative only. Infact, this is not a desirable implementation for a grounded appliancesince no effective ground protection is then afforded. When groundprotection is itself an important consideration, the circuit must havemuch lower sensitivity so as to operate with a leakage current in theorder of 0.9 mA. This can best be achieved by using two circuits: onehaving a virtual ground serving as a ground connection for the applianceand being calibrated for ground fault connection; and the other havingvery high sensitivity and serving as a touch switch as explained above.

Reference is now made to FIG. 4 showing a touch switch depictedgenerally as 35 comprising normally open first and second switchcontacts 81 and 82. The first switch contact 81 is connected to anincoming neutral feeder 83 of an a.c. main supply whose live feeder 84is connected to an input 85 of a d.c. comparator 86, as described abovewith reference to FIGS. 1 and 3 of the drawings. Connected to an outputof the comparator 86 is the base of an NPN bipolar junction transistor87 whose emitter is connected to the d.c. zero voltage rail and whosecollector is coupled to a relay 88, which may be a contactor forinterrupting current to an a.c. appliance. A capacitor 89 is connectedat the output of the comparator 86 for smoothing the comparator outputvoltage which serves as the base bias voltage of the transistor 87.

The touch switch 35 operates as follows. Under normal circumstances, theimpedance between the two switch contacts 81 and 82 exceeds thethreshold of 500MΩ and the switch is “open circuit”. When the impedanceacross the two switch contacts falls below this value, a small a.c.current in the order of several nanoamperes flows into the input 85 ofthe comparator 86 causing it to oscillate. That is, the output voltageacross the capacitor 89 goes HIGH and LOW The circuit is calibrated sothat, when this happens, the net voltage across the capacitor 89 issufficient to push the transistor 87 into saturation, thereby operatingthe relay 88.

It should also be noted that, in practice, the first switch contact 81may be connected to a virtual ground connection as explained above suchthat finger contact with the second switch contact 82 causes aneffective “short” between the two switch contacts, within the context ofthe invention. That is to say, the impedance between the two switchcontacts will fall from several hundred MΩ to less than 1MΩ.

FIG. 5 is a schematic representation of an application using low costsurface wiring for use with the touch switch shown in FIG. 4. Thus thereis shown a touch switch 35 for remotely operating an electricalappliance 29 as explained above with reference to FIG. 4 of thedrawings. The touch switch 35 is connected to a switch contact 92 via anelectrically conductive track 93. Touching the switch current 92 causesa “ground fault” as explained above, thereby operating the touch switch35 and interrupting current to the electrical appliance 29. Theelectrically conductive track 93 may be a graphite track and may even behand drawn using a pencil on the exterior surface of a wall (or anyother surface) on which the touch switch 35 is mounted. Alternatively,wallpaper (constituting a surface-covering) may have printed on asurface thereof a conductive track so that by abutting several pieces ofwallpaper with the electrical tracks in mutual contact, the effectivelength of the conductive track can be extended. Preferably, theconductive track is printed on the backside of the wallpaper and theswitch contact 92 may be push-fitted though the wallpaper at a desiredlocation so as to effect electrical contact with the hidden track. Insimilar manner, several switch contacts may be connected to the sametrack, so as to allow operation of the touch switch 35 from more thanone location via a suitably modified wallpaper, carpet or othersurface-covering.

The invention also contemplates that the switching circuit is an adapterfor coupling the appliance to an electrical socket outlet. Likewise, itmay be formed integral with an electrical socket outlet or an electricalplug or with the appliance.

What is claimed is:
 1. A switching circuit for use with an electricalappliance operating from an a.c. supply having live and neutralconnections and a reliable ground connection, the switching circuitcomprising: a main feeder interruption means operatively coupled to animpedance measuring circuit and being responsive to the switching signalfor opening a switching device connected in at least one of the live andneutral connections, first and second normally open switching contacts,and wherein said high sensitivity impedance measuring circuit is coupledto the first and second switch contacts for measuring an impedancetherebetween, said high sensitivity impedance measuring circuitincluding a differential comparator circuit for comparing a fraction ofthe voltage between the live and neutral connections with a voltage atthe first or second switch terminal of the switching circuit andproducing at an output thereof a switching signal if a differencebetween said voltages exceeds a predetermined threshold; thedifferential comparator circuit comprising: a first input connected tothe ground connection of the a.c. supply, and a second input connectedto a common junction of a voltage divider including a pair of resistorsconnected in series between the live and neutral terminals of the supplyfeeder; the appliance being coupled to said output so as to beresponsive to said switching signal; and the first switch contact beingelectrically floating relative to a ground feeder of the electricalsupply, so that a “ground fault” between ground and the first switchcontact induces the switching signal.
 2. The switching circuit accordingto claim 1, wherein the electrical appliance operates from an a.c.supply having live and neutral connections and there is furtherincluded: a main feeder interruption means operatively coupled to theimpedance measuring circuit and being responsive to the switching signalfor opening a switching device connected in at least one of the live andneutral connections.
 3. The switching circuit according to claim 2,wherein the main feeder interruption means is a contactor.
 4. Theswitching circuit according to claim 2, wherein the main feederinterruption means is an earth leakage circuit breaker.
 5. The switchingcircuit according to claim 2, wherein the impedance measuring circuitcomprises: a differential comparator circuit for comparing a fraction ofthe voltage between the live and neutral connections with a voltage atthe first or second switch contact of the switching circuit andproducing the switching signal if a difference between said voltagesexceeds a predetermined threshold.
 6. The switching circuit according toclaim 1, wherein the main feeder interruption means includes a normallyopen switching circuit coupled to said switching device connected in atleast one of the live and neutral connections and being responsive tothe switching signal for closing the switching device.
 7. The switchingcircuit according to claim 6, wherein the differential comparatorcircuit includes at least two comparators having respective outputswhich are commonly connected to said actuator.
 8. The switching circuitaccording to claim 7, further including at least one indication lamp forindicating a state of the actuator.
 9. The switching circuit accordingto claim 8, wherein the main feeder interruption means is connected to acollector of a bipolar junction transistor having a base thereofconnected to the normally open indication switch contact so as to beconnected to the supply rail of the d.c. supply when the actuator isenergized.
 10. The switching circuit according to claim 1, wherein themain feeder interruption means includes a normally closed switchingcontrol coupled to said switching device connected in at least one ofthe live and neutral connections and being responsive to the switchingsignal for opening the switching device.
 11. The switching circuitaccording to claim 1, wherein the main feeder interruption meansincludes a normally closed switching control coupled to said switchingdevice connected in at least one of the live and neutral terminals andbeing responsive to the switching signal for opening the switchingdevice.
 12. The switching circuit according to claim 1, wherein the mainfeeder interruption means includes a bistable switching circuit coupledto said switching device which is responsive to the switching signal foralternatively opening and closing the switching device.
 13. Theswitching circuit according to claim 1, wherein the differentialcomparator circuit is an integrated circuit.
 14. The switching circuitaccording to claim 1, being an adapter for coupling the appliance to anelectrical socket outlet.
 15. The switching circuit according to claim1, being integral with an electrical socket outlet.
 16. The switchingcircuit according to claim 1, being integral with an electrical plug.17. The switching circuit according to claim 1, being integral with theappliance.
 18. The switching circuit according claim 1, wherein thepredetermined threshold is selectable.
 19. The switching circuitaccording to claim 18, wherein the predetermined threshold is selectableand the impedance measuring circuit comprises: a differential comparatorcircuit connected as an open loop amplifier being calibrated to operateat its stability threshold so that injection of an a.c. current to aninput thereof pushes the amplifier into saturation thereby producing theswitching signal.
 20. A control system for switching an electricalappliance operating from an a.c. supply having live and neutralconnections, said control system comprising: a switching circuitincluding: first and second normally open switch contacts, and a highsensitivity impedance measuring circuit coupled to the first and secondswitch contacts for measuring an impedance therebetween, said highsensitivity impedance measuring circuit producing at an output thereof aswitching signal if said impedance is lower than a predeterminedthreshold; the first switch contact being electrically floating relativeto a ground feeder of an electrical supply, so that a “ground fault”between ground and the first switch contact induces the switchingsignal, said switching circuit being operatively coupled to theelectrical appliance for supplying or interrupting electrical powerthereto, and an auxiliary switch contact being connected by anelectrical conductor to the second switch contact of the switchingcircuit so that an impedance between the auxiliary switch contact andthe first switch contact less than a predetermined threshold producesthe switching signal; the second switch contact being mounted on asurface of a room and the electrical conductor being associated with asurface-covering for applying to said surface, so as to allow extendedoperation of the switching circuit within the room remote from thesecond switch contact.
 21. The control system according to claim 20,wherein the electrical conductor is formed by marking with anelectrically conductive material on said surface.
 22. A switchingcircuit for use with an electrical appliance operating from an a.c.supply having live and neutral connections and an unreliable groundconnection, the switching circuit comprising: a main feeder interruptionmeans operatively coupled to the impedance measuring circuit and beingresponsive to the switching signal for opening a switching deviceconnected in at least one of the live and neutral connections, first andsecond normally open switch contacts, and a high sensitivity impedancemeasuring circuit coupled to the first and second switch contacts formeasuring an impedance therebetween, said high sensitivity impedancemeasuring circuit including a differential comparator circuit forcomparing a fraction of the voltage between the live and neutralconnections with a voltage at the first or second switch terminal of theswitching circuit and producing at an output thereof a switching signalif a difference between said voltages exceeds a predetermined threshold;said impedance is lower than a predetermined threshold; the differentialcomparator circuit comprising: a first input connected to a virtualground connection which is electrically floating relative to the groundconnection of the a.c. supply, and a second input connected to a commonjunction of a voltage divider including a pair of resistors connected inseries between the live and neutral terminals of the supply feeder, theappliance being coupled to said output so as to be responsive to saidswitching signal; and the first switch contact being electricallyfloating relative to a ground feeder of an the electrical supply, sothat a “ground fault” between ground and the first switch contactinduces the switching signal.
 23. The switching circuit according toclaim 22, wherein the first switch terminal is electrically floatingrelative to a ground feeder of an electrical supply, so that a “groundfault” between ground and the first switch terminal induces theswitching signal.
 24. The switching circuit according to claim 22,wherein the main feeder interruption means is a contactor.
 25. Theswitching circuit according to claim 22, wherein the main feederinterruption means is an earth leakage circuit breaker.
 26. Theswitching circuit according to claim 22, further comprising: an actuatoroperatively coupled to an output of the differential comparator andconnected in series with the main feeder interruption means.
 27. Theswitching circuit according to claim 26, wherein the differentialcomparator circuit includes at least two comparators having respectiveoutputs which are commonly connected to said actuator.
 28. The switchingcircuit according to claim 27, further including at least one indicationlamp for indicating a state of the actuator.
 29. The switching circuitaccording to claim 28, wherein the main feeder interruption means isconnected to a collector of a bipolar junction transistor having a basethereof connected to the normally open indication switch contact so asto be connected to the supply rail of the d.c supply when the actuatoris energized.
 30. The switching circuit according to claim 22, whereinthe main feeder interruption means includes a normally open switchingcircuit coupled to said switching device connected in at least one ofthe live and neutral terminals and being responsive to the switchingsignal for closing the switching device.
 31. The switching circuitaccording to claim 22, wherein the main feeder interruption meansincludes a normally closed switching circuit coupled to said switchingdevice connected in at least one of the live and neutral terminals andbeing responsive to the switching signal for opening the switchingdevice.
 32. The switching circuit according to claim 22, wherein themain feeder interruption means includes a bistable switching circuitcoupled to said switching device which is responsive to the switchingsignal for alternatively opening and closing the switching device. 33.The switching circuit according to claim 22, wherein the differentialcomparator circuit is an integrated circuit.
 34. The switching circuitaccording to claim 22, being an adapter for coupling the appliance to anelectrical socket outlet.
 35. The switching circuit according to claim22, being integral with an electrical socket outlet.
 36. The switchingcircuit according to claim 22, being integral with an electrical plug.37. The switching circuit according to claim 22, being integral with theappliance.
 38. The switching circuit according to claim 22, wherein thepredetermined threshold is selectable.
 39. The switching circuitaccording to claim 22, wherein the predetermined threshold is selectableand the impedance measuring circuit comprises: a differential comparatorcircuit connected as an open loop amplifier being calibrated to operateat its stability threshold so that injection of an a.c. current to aninput thereof pushes the amplifier into saturation thereby producing theswitching signal.